Growth of gallium nitride crystals

ABSTRACT

The size and photoluminescent efficiency of crystals of gallium nitride, grown in a solution of molten gallium and bismuth, are improved by maintaining the partial pressure of ammonia vapor in a hydrogen gas atmosphere flowing above the solution at a value which is, at most, about an order of magnitude greater than the &#39;&#39;&#39;&#39;equilibrium pressure&#39;&#39;&#39;&#39; of formation vs. decomposition of gallium nitride. The photoluminescent efficiency of a blue band, whose energy is centered around 4,350 angstroms, emitted by such gallium nitride crystals is also improved by introducing zinc vapor into the carrier gas.

[lite States Patent [191 Logan et a1.

[451 Aug. 13, 1974 GROWTH OF GALLIUM NITRIDE CRYSTALS [75] Inventors:Ralph Andre Logan; Carl Dryer Thurmond, both of Morristown, NJ.

[73] Assignee: Bell Telephone Laboratories,

Incorporated, Murray Hill, NJ.

[22] Filed: Mar. 24, 1972 [21] Appl. No.: 237,896

[52] US. Cl. 423/409, 252/623 GA [51] Int. Cl ..C01h 21/06 [58] Field ofSearch 423/409; 75/134 T;

[56] References Cited UNITED STATES PATENTS 12/1968 Gershenzon et a1.252/623 GA OTHER PUBLICATIONS Johnson et al.: Journal of PhysicalChemistry, Vol.

36, (1932), p. 2,652. Maruska et a1.: Applied Physics Letters, Vol. 15,No. 10, Nov. 15, 1969, pp. 327-329.

Primary ExaminerOscar R. Vertiz Assistant ExaminerHoke S. MillerAttorney, Agent, or Firm-D. I. Caplan [5 7] ABSTRACT The size andphotoluminescent efficiency of crystals of gallium nitride, grown in asolution of molten gallium and bismuth, are improved by maintaining thepartial pressure of ammonia vapor in a hydrogen gas atmosphere flowingabove the solution at a value which is, at most, about an order ofmagnitude greater than the equilibrium pressure of formation vs.decomposition of gallium nitride. The photoluminescent efficiency of ablue band, whose energy is centered around 4,350 angstroms, emitted bysuch gallium nitride crystals is also improved by introducing zinc vaporinto the carrier gas.

10 Claims, 1 Drawing Figure 1 GROWTH OF GALLIUM NITRIDE CRYSTALS FIELDOF THE INVENTION This invention relates to methods for growingluminescent crystals and, more particularly, to a technique for thegrowth of photoluminescent gallium nitride crystals.

BACKGROUND OF THE INVENTION The development of technologies usingoptical displays and optical sources, such as the computer andcommunication technologies, has necessitated a search for light-emittingdevices which can emit light at certain desired visible wavelengths orcombinations thereof (colors). It has long been known thatsemiconductive gallium nitride can emit visible blue light whenirradiated with the invisible ultraviolet. Thus, blue photoluminescentsemiconductor devices, that is, devices emitting blue light underexcitation by an optical source, can be made using semiconductivegallium nitn'de crystals as the photoluminescent material (phosphor).

It has also been known in the prior art that gallium nitride crystalscan be formed from molten gallium by passing ammonia at atmosphericpressure over the molten gallium at a temperature of the order of 1,000"C. See: W. C. Johnson et al., Journal of Physical Chemistry, Vol. 36, p.2651 (1932). However, crystals of gallium nitride formed by such methodsare limited to a particle size of about a micron. See: H. G. Grimmeiss,Journal Applied Physics, Vol. 41, p. 4054 (1960). Crystals of such asmall size are not very useful for devices.

On the other hand, in the growth of gallium phosphide crystals fromgallium solutions, an improvement in crystal size has been reported byadding bismuth t the molten gallium growth solution, in order to inhibitthe formation of spurious gallium phosphide crystal nuclei and therebyto allow more raw material to be available for the growth of fewer butlarger gallium phosphide crystals. See: F. A. Trumbore et al., AppliedPhysics Letters, Vol. 9, p. 4 (1966). It has been thought in the art,however, that any growth of gallium nitride crystals from a solution,similar to that just mentioned for gallium phosphide, cannot possiblyyield reasonably large crystals, since the amount of gallium nitridewhich can be dissolved in the solution was believed to be too small; Itwould therefore be desirable to have available a solution growth methodfor growing gallium nitride crystals of considerably larger size thanpreviously obtained.

SUMMARY OF THE INVENTION We have discovered a method by which galliumnitride crystals can be grown in solution to a crystal size of the orderof l centimeter square by several mils thick or more, on a substratesuch as sapphire. In order to achieve this relatively large size growth,we have utilized a modified solution growth technique in which a galliumnitride crystal is grown on-the substrate immersed'in a heated growthsolution of molten gallium and bismuth, in the presence of ammonia vaporabove the solution in a furnace. We have found that it is important thatthe rate of reaction of ammonia with gallium be slowed by maintainingthe partial pressure of the ammonia vapor above the-solution at avalue'which is much lower than used in the prior art. The mainconsequence of this slower reaction rate is the elimination of theundesired spreading of solution which occurs at higher ammonia vaporpressures, whereby the liquid gallium flows up and over the walls of thecontainer (boat) into the furnace.

In setting forth the range of the partial pressure of ammonia vapor inhydrogen gas to be used in accordance with this invention, it isconvenient to define this range in terms of the equilibrium pressure atwhich the competing processes, of formation of gallium nitride fromgallium vs. decomposition of gallium nitride by hydrogen, arecharacterized by equal rates. In these terms, we have found that thepartial pressure of ammonia vapor should be maintained in the range fromslightly greater than this equilibrium pressure to less than about five,and at most ten times this equilibrium pressure, and preferably abouttwice this equilibrium pressure.

In a specific embodiment of the invention, ammonia vapor in a carriergas, hydrogen, is passed over the surface of a growth solution ofgallium and bismuth at an elevated temperature. Advantageously, zincvapor is also introduced into the carrier gas, in order to incorporatezinc as an impurity into the gallium nitride crystals and thereby toimprove the photoluminescent efficiency. Immersed in the growth solutionare substrates, such as sapphire, for the epitaxial crystal growth ofgallium nitride. A temperature gradient is maintained across the surfaceof the growth solution, such that the upstream end of the growthsolution with respect to the flowing carrier gas is at a somewhat highertemperature than the downstream end. Thereby, while nitrogen from theammonia vapor will continuously dissolve into the growth solution,mainly at the higher temperature (upstream) region of the solution, agrowth of gallium nitride crystals on the substrates will tend to occurin the lower temperature (downstream) region. This crystal growth thusoccurs in a controllable fashion in which the partial pressure ofammonia vapor in the carrier gas is continuously maintained at asuitable value as set forth previously.

Crystals of gallium nitride grown in accordance with our invention haveexhibited as much as 25 percent (photoluminescent efficiency) conversionat room temperature of ultraviolet'light from a nitrogen gas laser(3,500 angstroms) into visible blue light (ranging from about 3,800 to6,200 angstroms). Such crystals are also expected to be useful as thelasing material in solid state laser sources of blue light.

BRIEF'DESCRIPTION OF THE DRAWING This invention can be better understoodfrom the following detailed description when read in conjunction withthe drawing in which the FIGURE shows gallium nitride crystals beinggrown in a furnace, partly in perspective, in accordance with a specificembodiment of the invention.

DETAILED DESCRIPTION As shown in the FIGURE, a furnace 10 is maintainedat a temperature profile indicated immediately beneath it, byconventional heating means (not shown). It should be understood that theprecise temperatures indicated in the FIGURE are not crucial, but canvary over wide ranges so long as the overall general shape ofthe-profile is maintained. The furnace 10 has an inlet l1 and an outlet12 for the flow of ambient gas. At the inlet, this gas is composed ofammonia vapor in the carrier gas, hydrogen, at a total pressure of 1atmosphere. Toward the downstream end of the furnace is located a carbonheat sink l3 partially surrounding a growth boat 14; whereas toward theupstream of this furnace 10 is located a zinc dopant boat 15. In thisway, doping impurities of zinc are vaporized from the dopant boat andthen flow with the ambient gas over the growth boat 14.

By way of illustration only, typical dimensions for the various elementsand their mutual spacing are as follows. The growth boat 14 is aboutthree inches long in the x direction, one-half inch wide in the zdirection, and one-half inch deep in the y direction. The heat sink 13is about four inches long in the x direction and is about three-quarterinch wide in the z direction, and has a recess about one inch long inthe x direction and one-half inch wide in the z direction, in order toaccommodate a portion of the growth boat 14 in close contact therewith.About 12 inches to the left-hand side of the growth boat 14 and at adistance of about two inches from the inlet side of the furnace 10, thedopant boat 15 is located. This dopant boat 15 is about three incheslong in the x direction, one-half inch wide in the 2 direction, andone-half inch deep in the y direction. The furnace 10 is in the form ofa cylinder having a crosssection diameter of about seven-eighths inch.This furnace l0,as well as the growth boat 14 and the dopant 'boat 15,can be made of pyrolitic carbon or quartz, for

example.

In order to grow gallium nitride crystals, 21 growth solution alloy ofgallium and bismuth is introduced into the growth boat 14 to fill theboat typically about ninetenths full. Then, sapphire substrates orientedtypically (000i) are placed on the top surface of the solution in thegrowth boat 14. Next, these substrates 21 are covered with more solutionof gallium and bismuth so that the growth solution in the growth boat 14fills this boat, thereby leaving these substrates 21 suspended in thegrowth solution. Each of these substrates 2] has a cross section ofapproximately 1 cm in the X2 plane and advantageously has been coatedwith a predeposit of a gallium nitride epitaxial layer on the surfacewhere gallium nitride is to be grown. For example, well-known vaporphase reactions of the prior art can be used for this predeposit,typically three microns thick. However, it should be understood thatthis pre-deposit is not necessary, but such a predeposit is useful innucleating growth so that a gallium nitride crystal is grown over theentire predeposited region of the substrate rather than a major fractionthereof.

The temperature profile indicated in the temperature profile in thedrawing is then established in which the average temperature of thegrowth alloy is in the range of about 850 C. to l,050 C., typicallyabout l,000 C. The left-hand end of the growth boat 14 is maintained ata temperature gradient corresponding to a temperature diflerence ofabout 75 C. over a distance of about 3 inches in the x direction(allowing for the 1 inch recess in the'carbon heat sink 13). Thistemperature difference is not critical and can vary by about 50C., andthus the corresponding temperature gradient can be in. the range ofabout 17 C. to 33 C. per inch. The dopant boat 15 is maintained at atemperature in the range of about 500 C. to about 825 C. and containsmolten zinc 15.5. Thereby, zinc vapor at partial pressures of betweenapproximately 0.001 and 0.1 atmosphere is introduced into the carriergas of hydrogen containing ammonia vapor, flowing from the inlet 1 l.The rate of gas flow from inlet 11 to outlet 12 is maintained at a rateof typically 140 cm per minute. With the temperature profile maintainedas indicated in the FIGURE, gallium nitride then grows a single crystallayer on the sapphire substrates 2] to a thickness of a few mils in atime period of about 16 hours.

The solution in the growth boat 14 contains gallium and bismuthtypically in a 50/50 ratio by atomic percent. For this 50/50 mixture,advantageously the partial pressure of ammoniaat the inlet 11 isadjusted to be in the range of about 3 X 10 and l X 10" atmospheres,typically 6 X 10' atmospheres. However, a different ratio of bismuth togallium also can be used in this invention. In particular, this ratiocan be as high as about 90 atomic percent bismuth in combination withusing a somewhat higher partial pressure of ammonia (0.03 atmosphere) inthe hydrogen gas flowing into the inlet pipe 11. Likewise, as little as30 atomic percent bismuth in gallium solution may be used in combinationwith a partial pressure of ammonia of about 4 X 10 atmosphere in theflowing hydrogen gas.

It should be recognized that higher concentrations of bismuth in thegallium solution in the boat 14 correspond to a greater ammonia partialpressure in the carrier gas flowing from the inletpipe 11 to the outletpipe 12, in order to maintain the same conditions relative toequilibrium conditions (rate of decomposition in hydrogen equal to therate of formation from gallium of gallium nitride), according to thewell-known stoichiometric relations:

Gallium nitride semiconductor crystals grown by the above-describedmethod are characterized by n-type conductivity. Utilizing thesecrystals grown by the above-described method, conversion of at least 25percent of the output radiation of a nitrogen gas laser (about 3,500 A)into visible blue light has been achieved.

While this invention has been described in detail in terms of a specificembodiment, various modifications may be made without departing from thescope of this invention. 'For example, inhibitors other than bismuth canbe used in the molten gallium growth solution, such as antimony, lead,tin, thallium, and indium. In addition, instead of the use of sapphiresubstrate, other substrates can be used, such as silicon carbide orother substrates with lattice structures compatible with the growth ofgallium nitride. Moreover, it should be recognized by the skilled workerthat as there becomes available in the art a method for making p-typegallium nitride crystals, then such crystals could be used as thesubstrate for making p-n junctions by the abovedescribed method inaccordance with the invention. Such p-n junctions would be especiallyuseful for making electroluminescent devices, that is, light-emittingdiodes excited by electrical current. Alternatively, various surfacebarriers, such as Schottky barriers formed by such metals as gold oraluminum, on the n-type gall. A method of growing a crystal of galliumnitride which comprises the steps of:

a. placing a substrate of sapphire in a solution comprising moltengallium which contains bismuth as an inhibitor of the growth of galliumnitride, at a temperature in the range of between about 850C and aboutl,050C; and

b. flowing a gas mixture comprising ammonia vapor in hydrogen gas acrossthe exposed surface of the solution, said ammonia vapor being maintainedat a predetermined partial pressure which is no more than about an orderof magnitude greater than the equilibrium partial pressure of ammoniavapor with respect to the formation vs. decomposition of gallium nitridein said solution, whereby a single crystal layer of gallium nitride isgrown on the substrate in the solution.

2. The method recited in claim 1 in which the composition of thesolution is in the range between about atomic percent to about 90 atomicpercent bismuth in gallium, and the partial pressure of ammonia vapordoes not exceed 0.03.

3. The method recited in claim 1 in which a temperature gradient isestablished across the exposed surface of the solution, said temperaturegradient being in the range of about 17 to about 33 C. per inch.

4. The method recited in claim 1 in which the said partial pressure ofthe ammonia vapor is no more than about five times the said equilibriumpartial pressure.

5. The method recited in claim 4 in which the partial pressure ofammonia vapor is about twice the equilibrium pressure.

6. The method recited in claim 1 in which the substrate has a predepositthereon of epitaxial gallium nitride.

7. The method recited in claim 1 which further includes introducing zincvapor into the gas mixture.

8. The method recited in claim 7 in which the partial pressure of zincvapor is of the order of 0.1 atmo sphere.

9. A method of growing a crystal of gallium nitride which comprises:

a. placing a substrate of sapphire in a solution comprising moltengallium and an inhibitor of the growth of gallium nitride, at atemperature in the range of between about 850C and about l,050C, saidinhibitor being essentially a member of the group consisting of bismuth,antimony, tin, thallium and indium;

b. flowing a gas mixture containing ammonia vapor in hydrogen across theexposed surface of the solution, said ammonia vapor being at apredetermined partial pressure no more than five times the equilibriumpressure for the formation vs. decomposition of gallium nitride, wherebya single crystal layer of gallium nitride is grown on the substrate inthe solution.

10. The method of claim 9 in which a temperature gradient is maintainedin the range of about 17 to about 33C per inch across the exposedsurface of the solution during the step of flowing the gas mixture.

2. The method recited in claim 1 in which the composition of thesolution is in the range between about 10 atomic percent to about 90atomic percent bismuth in gallium, and the partial pressure of ammoniavapor does not exceed 0.03.
 3. The method recited in claim 1 in which atemperature gradient is established across the exposed surface of thesolution, said temperature gradient being in the range of about 17* toabout 33* C. per inch.
 4. The method recited in claim 1 in which thesaid partial pressure of the ammonia vapor is no more than about fivetimes the said equilibrium partial pressure.
 5. The method recited inclaim 4 in which the partial pressure of ammonia vapor is about twicethe equilibrium pressure.
 6. The method recited in claim 1 in which thesubstrate has a predeposit thereon of epitaxial gallium nitride.
 7. Themethod recited in claim 1 which further includes introducing zinc vaporinto the gas mixture.
 8. The method recited in claim 7 in which thepartial pressure of zinc vapor is of the order of 0.1 atmosphere.
 9. Amethod of growing a crystal of gallium nitride which comprises: a.placing a substrate of sapphire in a solution comprising molten galliumand an inhibitor of the growth of gallium nitride, at a temperature inthe range of between about 850*C and about 1,050*C, said inhibitor beingessentially a member of the group consisting of bismuth, antimony, tin,thallium and indium; b. flowing a gas mixture containing ammonia vaporin hydrogen across the exposed surface of the solution, said ammoniavapor being at a predetermined partial pressure no more than five timesthe equilibrium pressure for the formation vs. decomposition of galliumnitride, whereby a single crystal layer of gallium nitride is grown onthe substrate in the solution.
 10. The method of claim 9 in which atemperature gradient is maintained in the range of about 17* to about33*C per inch across the exposed surface of the solution during the stepof flowing the gas mixture.